From 9a0a53354e2f51a2acb2a1160209c8fca781f81e Mon Sep 17 00:00:00 2001
From: maksim bondarenko <maksim.bondarenko@sap.com>
Date: Mon, 9 Feb 2026 16:10:06 +0200
Subject: [PATCH 1/2] create package

---
 .../20-nil-interface-gotcha/myerror.go        | 38 +++++++
 .../20-nil-interface-gotcha/myerror_test.go   | 99 +++++++++++++++++++
 2 files changed, 137 insertions(+)
 create mode 100644 04-errors-semantics/20-nil-interface-gotcha/myerror.go
 create mode 100644 04-errors-semantics/20-nil-interface-gotcha/myerror_test.go

diff --git a/04-errors-semantics/20-nil-interface-gotcha/myerror.go b/04-errors-semantics/20-nil-interface-gotcha/myerror.go
new file mode 100644
index 0000000..d7ddd1f
--- /dev/null
+++ b/04-errors-semantics/20-nil-interface-gotcha/myerror.go
@@ -0,0 +1,38 @@
+package myerror
+
+import (
+	"fmt"
+)
+
+type MyError struct {
+	Op string
+}
+
+func (e *MyError) Error() string {
+	if e == nil {
+		return "<nil MyError>"
+	}
+	return fmt.Sprintf("operation failed: %s", e.Op)
+}
+
+// BuggyDoThing returns a *MyError(nil) as error (typed nil pitfall)
+func BuggyDoThing(fail bool) error {
+	if fail {
+		return &MyError{Op: "fail"}
+	}
+	var e *MyError = nil
+	return e // BAD: interface is non-nil!
+}
+
+// FixedDoThing returns a true nil error when no error
+func FixedDoThing(fail bool) error {
+	if fail {
+		return &MyError{Op: "fail"}
+	}
+	return nil // GOOD: interface is nil
+}
+
+// Wraps error for errors.As demonstration
+func WrapError(err error) error {
+	return fmt.Errorf("wrapped: %w", err)
+}
diff --git a/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go b/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go
new file mode 100644
index 0000000..4f78a21
--- /dev/null
+++ b/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go
@@ -0,0 +1,99 @@
+package myerror
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestDoThingMatrix(t *testing.T) {
+	tests := []struct {
+		name     string
+		fn       func(bool) error
+		fail     bool
+		wantNil  bool
+		wantType string
+	}{
+		{
+			name:     "BuggyDoThing returns typed nil (should NOT be nil)",
+			fn:       BuggyDoThing,
+			fail:     false,
+			wantNil:  false,
+			wantType: "*myerror.MyError",
+		},
+		{
+			name:     "BuggyDoThing returns error (fail=true)",
+			fn:       BuggyDoThing,
+			fail:     true,
+			wantNil:  false,
+			wantType: "*myerror.MyError",
+		},
+		{
+			name:     "FixedDoThing returns nil (should be nil)",
+			fn:       FixedDoThing,
+			fail:     false,
+			wantNil:  true,
+			wantType: "<nil>",
+		},
+		{
+			name:     "FixedDoThing returns error (fail=true)",
+			fn:       FixedDoThing,
+			fail:     true,
+			wantNil:  false,
+			wantType: "*myerror.MyError",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.fn(tt.fail)
+			gotNil := err == nil
+			gotType := "<nil>"
+			if err != nil {
+				gotType = fmt.Sprintf("%T", err)
+			}
+			if gotNil != tt.wantNil {
+				t.Errorf("expected nil: %v, got: %v (type: %s, value: %#v)", tt.wantNil, gotNil, gotType, err)
+			}
+			if gotType != tt.wantType {
+				t.Errorf("expected type: %s, got: %s", tt.wantType, gotType)
+			}
+		})
+	}
+}
+
+func TestErrorsAsExtractionMatrix(t *testing.T) {
+	tests := []struct {
+		name     string
+		wrap     bool
+		expectOp string
+	}{
+		{
+			name:     "Direct MyError",
+			wrap:     false,
+			expectOp: "extract",
+		},
+		{
+			name:     "Wrapped MyError",
+			wrap:     true,
+			expectOp: "extract",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			orig := &MyError{Op: "extract"}
+			var err error = orig
+			if tt.wrap {
+				err = WrapError(err)
+			}
+			var me *MyError
+			if !errors.As(err, &me) {
+				t.Fatalf("errors.As failed to extract *MyError")
+			}
+			if me == nil || me.Op != tt.expectOp {
+				t.Fatalf("extracted wrong value: %#v", me)
+			}
+		})
+	}
+}

From 91c062b174f20e335327b3ef57f60aa385c3a86c Mon Sep 17 00:00:00 2001
From: maksim bondarenko <maksim.bondarenko@sap.com>
Date: Mon, 23 Mar 2026 16:40:30 +0200
Subject: [PATCH 2/2] add test

---
 .../20-nil-interface-gotcha/myerror_test.go   | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go b/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go
index 4f78a21..fa2d4ce 100644
--- a/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go
+++ b/04-errors-semantics/20-nil-interface-gotcha/myerror_test.go
@@ -97,3 +97,31 @@ func TestErrorsAsExtractionMatrix(t *testing.T) {
 		})
 	}
 }
+
+func TestErrorMethodOutput(t *testing.T) {
+	tests := []struct {
+		name        string
+		err         *MyError
+		expectedMsg string
+	}{
+		{
+			name:        "nil receiver",
+			err:         nil,
+			expectedMsg: "<nil MyError>",
+		},
+		{
+			name:        "normal error",
+			err:         &MyError{Op: "test-operation"},
+			expectedMsg: "operation failed: test-operation",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			msg := tt.err.Error()
+			if msg != tt.expectedMsg {
+				t.Errorf("expected message: %q, got: %q", tt.expectedMsg, msg)
+			}
+		})
+	}
+}